Rechargeable batteries and charge control circuit therefore

ABSTRACT

A dry cell battery system comprising: a sealed battery capable of being repeatedly charged and discharged and including a sealed casing having exposed negative and positive terminals, positive and negative battery plates within said casing and electrically connected respectively to said positive and negative terminals, said negative battery plate having a negative potential in the range of about from minus 0.7 to minus 0.9 volts and an electrolyte within said casing forming with said positive and negative battery plates a rechargeable electro-chemical system which generates substantial quantities of hydrogen during the overdischarging thereof and during the high rate charging thereof at a low temperature, a circuit connected to the battery for operating the battery under one of said conditions which generates substantial quantities of hydrogen; and a porous platinum group metal electrode in said battery which is connected through a low impedance path to said negative terminal for absorbing said substantial quantities of hydrogen.

finite States H 1 1111 3,769,88

Seiger et al. Oct. 30, 1973 RECHARGEABLE BATTERIES AND [57] ABSTRACTCHARGE CONTROL CIRCUIT THEREFORE [75] Inventors: Harvey N. Seiger, EastBrunswick,

1.; Paul F. Rmerman, Huntington A dry cell battery system compnsmg: asealed battery capable of being repeatedly charged and discharged andincluding a sealed casing having exposed negative Asslgnee: Gum)"Industries, -9 Memchenv and positive terminals, positive and negativebattery plates within said casing and electrically connected re- [22]Filed: Aug 12, 1968 spectively to said positive and negative terminals,said negative battery plate having a negative potential in PP 752,024the range of about from minus 0.7 to minus 0.9 volts and an electrolytewithin said casing forming with said 52 U.S. c1. 136/3, 136 6 GC,136/179 Positiwa and negative battery Plates a rechargeable 51 1111. C1...H01m 35/00 mm-chemical System which generates Substantial 5s 1 Fieldof Search 136/3, 6, 20, 24, quantities 0f hydmge" during theverdischarging 136/179 thereof and during the high rate charging thereofat a low temperature, a circuit connected to the battery for [56]References Cited operating the battery under one of said conditionsUNITED STATES PATENTS which generates substantial quantities ofhydrogen; and a porous platinum group metal electrode in said 2. vbattery which is connected through a low impedance IC (3001'...3,350,225 10/1967 Seiger u 136/3 path to sa1d negative termlnal forabsorbing said sub 3,424,617 1 1969 Griegeretal.. 136 6stan'tialquantitiemfhydrogen- 3,438,812 4/1969 Chemey 6:31. 136 6Primary Examiner-A. B. Curtis 2 Claims, 6 Drawing Figures AssistantExaminer-C. F. LeFevour Attorneywallenstein, Spangenberg, Hattis andStrampel 1' "a 2 I z l f 23 I 7 CURRENT y lo ssrvsmc CHARGE I8 7 MEANSZZQ' CONTROL MEANS 4 l cHAeE 12 i 332 /Z4 VOLTAGE I 22 sou cs 8 bPATENTH] UB1 3 0 I975 SHEET 2 [1F 2 RECHARGEABLE BATTERIES AND CHARGECONTROL CIRCUIT THEREFORE The invention described herein was made in theperformance of work under a NASA contract and is subject to theprovisions of Section 305 of the National Aeronautics and Space Act of1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457.

The present invention relates to rechargeable sealed secondary dry cellbatteries and has its most important application in alkalinenickelcadmium and silvercadmium dry cell batteries. The presentinvention has its most important application in those rechargeablesealed dry cell batteries where hydrogen and/or oxygen may be generatedduring the charging or overcharging thereof.

In sealed rechargeable batteries of the type which generate substantialquantities of oxygen during any significant overcharging thereof, theuse of a relatively low charging current is usually recommended in theabsence of means for preventing the overcharge thereof. Oxygen isproduced during overcharge at a very slow rate at low charge rates andthe small amount of oxygen then generated is readily consumed at thenegative battery plates. However, at low charge rates the charging timeof batteries is frequently inconveniently long, taking as much as from 8to 12 hours. The substantial reduction of the charge time by usingabnormally high charge rates has not been heretofore achieved withconventional battery constructions in the absence of overchargepreventing means because the negative battery plates (cadmium hydroxidein a nickel cadmium battery) cannot consume fast enough the largequantities of oxygen generated during a substantial overcharge thereofat high charge rates to prevent the dangerous build-up of pressurewithin the batteries involved.

Also, at high charge rates and low temperatures, particularlytemperatures in the neightborhood of about C. and lower, substantialamounts of hydrogen are frequently generated from water because part ofthe normally excess amount of cadmium hydroxide used to prevent hydrogengeneration becomes chemically inert and all the reduceable cadmiumhydroxide becomes used up before a fully charged condition is reached.Therefore there is a substantial excess of hydrogen which must beabsorbed.

It has, heretorfore, been proposed to incorporate special oxygenconsuming electrodes in sealed rechargeable batteries connected to thenegative plates or terminals of the battery where they consumesubstantially all of the oxygen generated during the high rate chargingof the batteries. However, the oxygen consuming ability of many oxygenconsuming electrodes descreases with decrease in temperature. Where thephysical and/or electro-chemical conditions thereof cause thetemperature to fall substantially below room temperature, the oxygenconsuming ability of many of the oxygen consuming electrodes deteriorateso much that the volume of electrode material needed to absorb all ofthe generated oxygen at high charge rates may become prohibitive. Thisproblem can be at least partially alleviated by using a small oxygenconsuming electrode merely to detect the presence of oxygen generationoccurring during a small overcharge of the battery by sensing thecurrent flowing in the electrode and terminating the charge of thebattery when this current reaches a given trip level indicating only aslight overcharge of the battery. Oxygen consuming electrodes of thetype disclosed in U. S. Pat. No. 3,350,225 are especially suitable forthis purpose because the current flow therethrough varies appreciablywith the oxygen pressure of the battery to permit a proper selection ofcurrent to terminate charging at the desired point. lt has beendiscovered that modest oxygen generation takes place well ahead of thefully charged condition of the battery. Thus, to effectively fullycharge a battery by the detection of the current flow through the oxygenconsuming electrode, the magnitude of current flow necessary to operatethe sensing element which terminates the battery charging operation(i.e., the trip level) must be set to correspond to a point indicatingthe actual full or near fully charged condition of the battery involved.

This charge termination system proved unsatisfactory for applicationswhere hydrogen is generated during high rate charging at low temperaturebecause unsafe quantities of hydrogen are produced by the time the fullycharged condition of the battery is reached. Also, it was surprisinglydiscovered that some oxygen generation occurs even after charging of thebattery is terminated, so that once a battery charging operation hasbeen terminated, the current flow through the oxygen consuming electrodemay continue to exceed the trip level for a substantial period of timeso that, if the battery should be quickly discharged, it may not be ableto be effectively recharged in time to be used when needed again. Thisproblem is particularly acute in power systems for satellites where thebattery is charged by solar cells during the period the satellite ispassing through sunlight. It is manifest that the battery should becapable of being recharged whenever it passes through sunlight.

The present invention, among other things, provides a simple andreliable means for relatively quickly and efficiently absorbing hydrogengas generated in the battery during a rapid charge at low temperaturesor during an overcharge thereof and also for quickly and efficientlyabsorbing any modest but troublesome quantity of oxygen which remains oris generated in the battery after termination of a charging operation,so that the current flowing in the oxygen consuming electrode whichontrols the termination of the battery charging will quickly fall belowthe trip level after termination thereof.

The present invention utilizes a metal from the platinum group in theperiodic table, namely, platinum (which is decidely preferred), iridium,osmium, palladium, rhodium or ruthenium. Platinum, for example, has beenknown to be useful as a hydrogen gas absorbing material when connectedto a positive electrode of a battery through a resistor. Similarly,platinum has been known to be useful as an oxygen gas absorbingelectrode when connected to the negative terminal of a battery through aresistor. In accordance with the present invention, it has besurprisingly discovered that platinum in a-highly porous form hasexceedingly good hydrogen gas absorging characteristics when connectedthrough a very low impedance path to the negative terminal of a batterylike a nickel or silver cadmium battery where the negative cadmiumplates have a voltage of 0.8 volts. in accordance with the teachings ofthe prior art, it would not occur to one having ordinary skill in theart to operate a battery during charge or overdischarge so thatsubstantial quantities of hydrogen gas are generated which must beabsorbed by a platinum electrode connected to the negative terminal ofthe battery. Thus, it has been discovered that a porous platinumelectrode connected directly to the negative terminal of a sealed,rechargeable, dry cell battery permits the battery to be operated underconditions which generate hydrogen gas at a rate as high as 0.3milliters gas per minute of hydrogen gas per square inch of scavengerelectrode surface area. By its connection to the negative terminal ofthe battery through a low resistance path, the porous platinum electrodealso acts as an efficient oxygen gas absorbing electrode which canabsorb the aforementioned oxygen which remains and is generated in thebattery after termination of a charging operation thereof.

Although the use of platinum group metals operates most effectively whenconnected to cadmium negative battery plates, the broader aspects of theinvention contemplate the use of porous platinum group metal electrodeswith battery plate materials having potentials in the range of fromabout 0.7 volts to about 0.9 volts so metals like iron and cobalt areincluded.

The above and other advantages and features of the invention will beapparent from the specification to follow, the claims and the drawingswherein:

FIG. 1 is a simplified diagram of a battery charge system in which thebattery of the invention is particularly useful;

FIG. 2 is a diagram illustrating the operation of the battery system ofFIG. 1 utilized in a satellite where the batteries are charged fromsolar cells;

FIG. 3 is a vertical sectional view of a prismatic dry cell battery ofthe present invention which is utilized in the battery charge system ofFIG. 1;

FIG. 4 is a transverse sectional view of the battery of FIG. 3, takensubstantially along the line 44 therein;

FIG. 5 is a perspective view of the platinum electrode structure formingpart of the battery of FIGS. 3 and 4; and

FIG. 6 is a diagram illustrating the hydrogen absorbing quantities ofthe platinum electrode of the battery of FIGS. 3 and 4.

In FIG. 1, a typical chargeable sealed dry cell battery is indicated byreference numeral 1 and includes a sealed casing 4 in which two or moresets of positive and negative battery plates 6 and 8 are provided whichare connected through conductors respectively to positive and negativeterminals 10 and 12 on the casing 4. The battery casing also includesanoxygen consuming charge condition sensing electrode 16 which, in thealkaline battery being described, is made of porous nickel or silver(preferably nickel) free of all materials except for a thin layer ofelectrolyte. The oxygen consuming charge condition sensing electrode 16is connected to a terminal 18 on the outside of the casing whichterminal is connected through a conductor 17', current sensing means 20and a conductor 21 to the negative battery terminal 12. The electrolyteis preferably carried by absorbent layers 14 of an electrolyte absorbentmaterial. The electrode 16 is most advantageously separated from thebattery plates by layers l7-l7 of a perforated or apertured materiallike nylon netting which are immediately adjacent the electrode, andshort paths of electrolyte absorbent material formed by thin layers141-14 of an electrolyte absorbent material. In such case, the electrode16 remains wet with a thin layer of electrolyte indefinitely (i.e.,

they do not dry out) because there is a short path between the electrode16 and a source of water generation at the positive plates duringcharging of the battery. The casing 4 also contains one or more porousplatinum group metal electrodes 19 connected to the positive batteryplates 8 or negative casing terminal 12.

As previously indicated, the present invention has its most significantapplication in sealed nickel and silvercadmium dry cell batteries. Wherethe battery is a nickel-cadmium battery, each completely dischargedpositive plate 6 would most preferably comprise divalent nickel ornickelous hydroxide (Ni(OI-I) impregnated into a sintered porous nickelbase plate. In silvercadmium batteries, silver hydroxide is substitutedfor the nickelous hydroxide as the active material in the positiveplates. The completely discharged negative plates 8 in nickel orsi1ver-cadmium batteries each would most preferably be cadmiumimpregnated into a sintered porous nickel base plate. The electrolytewould most preferably be aqueous potassium hydroxide, such as a 30-34percent solution of same.

In FIG. 1, the source of charging current for the battery 1 isidentified by reference numeral 22, and where the invention is appliedto an earth satellite, the charging current source would be solar cellswhich generate a charging voltage when subjected to sunlight.

When the positive and negative terminal 10 and 12 of the battery 1 areconnected to the positive and negative terminals 22a and 22b of thecharging current source 22, as the degree of charge of the batteryreaches, say, 20-80 percent of a fully charged condition, oxygen isgenerated at the positive battery plates, the rate of the reactionincreasing as a fully charged condition is reached and exceeded. Untilthe fully charged condition of the battery is reached, the main reactiontaking place at the positive plates is the following:

Ni (OI-U OH" Ni OOI-l H O e (2) During overcharge of the battery, watercontinues to be generated at the positive plates by the followingreaction: I

The oxygen generated at the positive plates migrates to the oxygenconsuming electrodes where the following reaction takes place:

4I-I 0 2H O 4) where the rate of the reaction is proportional to oxygenpressure. The maintenance of a supply of the hydrogen atoms shown inequation (4) on the oxygen consuming electrodes 16 requires a source ofelectrons. These electrons are obtained from the negative battery platesduring charging of the battery by the oxidation of cadmium thereat asfollows:

2 Cd 4 OH 2 Cd (0H) 4e (5) The water in the electrolyte supplies thehydrogen atoms (as distinguished from hydrogen gas) and hydroxyl ions atthe surface of each oxygen consuming electrode by means of a corrosioncouple consisting of the Cd/Cd (OI-I), of the negative plates inelectrical contact with the nickel or silver of the oxygen consumingelectrodes. Depicted electro-chemically the corrot 92alh=-l where theconnecting line indicates contact between the nickel or silver areas.The reaction of the corrosion couple in reaction is the following:

4H O+4e 4H+4OH (6) Thus, when oxygen strips the surface of the oxygenconsuming electrodes of hydrogen atoms, the reactions (5) and (6) thentake place to supply new hydrogen atoms required by equation (4) and theOH ions required by equation (5).

The electrons in equation (5) must travel from the negative batteryplates 8 through an external electrical circuit to the oxygen consumingcharge condition sensing electrode 16 where they are consumed inreaction (6). As long as oxygen is present, the electrochemicalreactions (4), (5) and (6) represent a dynamic rate process where therate of electron flow (i.e., the current flow) in the current sensingmeans 20 in FIG. 1 will be proportional to oxygen pressure (i.e., theunconsumed oxygen) in the battery and is a measure of the degree ofcharge or overcharge of the battery.

It is believed that the ability of the above mentioned corrosion coupleto produce hydrogen atoms by equation (5), and hence absorb oxygenthereat, is a function of this closeness of the voltage of the oxygenconsuming electrode 16 to that of the negative battery plates which is-0.8 volts in an alkaline cell with cadmium as the negative activematerial.

At moderate temperatures, no hydrogen gas (H is given off during thecharging of the battery. However, at low temperatures (such as 0C. andlower), and relatively high charge rates (e.g., charge currents of atleast about two times the ampere hour rating of the battery at 0C. orone-half the ampere hour rating at 40C.), hydrogen gas is given off dueto the reaction:

The current sensing means 20 which, for example, may be a relay magneticamplifier or transistors circuit, controls a charge control means 23 inthe circuit between the charging current source 22 and the positive andnegative battery terminals and 12. The charge control means 23, may, forexample, be a set of relay contacts, an electronic switch or a magneticswitch. The current sensing means is designed to operate the chargecontrol means to decouple or disconnect the charging current source 22from the battery when the current flow between the negative batteryplates 8 and the oxygen consuming electrodes 16 is at a level, referredto as the trip level, which represents the fully charged condition ofthe battery which, as previously indicated, is generally a current levelwell in excess of the current level at the beginning of oxygengeneration.

When the battery is fully charged and a load device 24 is connectedbetween the battery terminals 10 and 12, the battery will discharge. lfoxygen generation would stop on discharge, as it would be expected todo, the current flow through the current sensing means 20 would startimmediately to decrease as the oxygen consumption of the oxygenconsuming electrode 16 proceeds. The charge circuit of the battery wouldthen soon become operative again as the current dropped below theaforesaid trip level (or a somewhat lower trip level where a relay isutilized because the pull-in current of a relay is generally somewhathigher than the drop-out level). However, oxygen generation surprisinglycontinues for a while after termination of a battery charge operation.The build-up of pressure within the casing 4 is prevented, however, bythe platinum electodes 22 which absorb the oxygen generated within thecasing and, in the absence of a fairly efficient oxygen absorbing means,as previously indicated, current above the trip level would continue toflow after termination of the charging operation. This oxygen could beabsorbed if the surface areas of the oxygen consuming charge conditionsensing electrodes 16 were utilized. However, it was discovered that, byproviding porous platinum electrodes 19 connected to the negativebattery plates 12 through a low impedance path,large amounts of bothhydrogen and oxygen can be quickly absorbed.

To illustrate what would happen in the absence of a good oxygenabsorbing means in the casing 4, reference should be made to thewaveform W1 in FIG. 2 which shows the variation in the current flowthrough the current sensing means 20 when the battery charged systemthereshown is utilized in an earth orbiting satellite, where thecharging current source 22 is one or more solar cells which operate togenerate a voltage only when the satellite is subjected to sunlight. InFIG. 2, in the time interval between T0 and T1 the satellite passesthrough a sunlighted portion of its orbit. During the next time intervalbetween T1 and T2, the satellite passes through a dark portion of itsorbit, and during the subsequent interval between T2 and T3 thesatellite again passes through a sunlighted portion of its orbit. [t isassumed that the battery 1 begins to discharge when the satellite passesthrough a dark portion of its orbitwhere the solar cells 22 cannotsupply current or voltage to operate the electrical equipment involved,and that the battery must be fully charged by the solar cells as thesatellite passes through a sunlit portion of its orbit. It is alsoassumed that the current level It indicated in FIG. 2 represents thecurrent flowing in the current sensing means 20 which indicates a fullycharged condition of the battery, that is a condition when all of thenickelous hydroxide of the negative battery plates of the exemplarybattery which can be oxidized has been oxidized for the charge currentinvolved and at which level it is desired to interrupt the charging ofthe battery. This is the trip level referred to previously.

It can be seen in FIG. 2 that in the time interval between T0 and Ta thecurrent flowing through the current sensing means 20 slowly increasesdue to the very slow generation of oxygen during the non-criticalportions of the charging cycle. However, at time Ta the rate of oxygengeneration and the current flow between the negative plates 8 and theoxygen consuming electrodes l8 suddenly increases, and at time Tb thelatter reaches the trip level It where the current sensing means 20operates the charge control means 23 which disconnects the solar cells22 from the battery 1. Due to the presence of what is believed to beunstable oxides, oxygen actually continues to be generated at a highrate even after disconnection of the solar cells from the battery, andwhere there is a limited oxygen consuming capability in the battery, asshown by waveform W1, the oxygen pressure and the current flow in thecurrent sensing means continues to increase for a while in the intervalbetween times Tb and Tc. A point is reached at the time Tc, however,when the limited rate of oxygen generation of the unstable oxidesgradually tapers off to a point where oxygen consumption is greater thanthe oxygen generation. Then the current in the current sensing means 20and the battery pressure starts to decrease gradually. A batterycharging operation cannot resume until the current in the currentsensing means 20 drops to the trip level It. The time it takes for thedecay of this current to the trip level is determined by the ability ofthe oxygen consuming electrodes 16 to consume the oxygen remaining inthe battery. As previousl indicated, when porous platinum group metalelectrodes w are utilized, oxygen (and hydrogen gas) absorption proceedsat such a fast rate that the waveform W1 of current flow in the currentsensing means drops almost immediately after charging ceases. However,in the absence of a good oxygen absorbing means like the platinumelectodes, the decay of the current starting at time Tc follows thedashed curve W2 where the current would not decay below the trip levelIt until the time Te occurring during the next dark period beginning attime T3. The entire sunlit period T2-T3 would then be unused forcharging the battery 1 so the battery would be unable to supply adequateelectrical power to operate the electrical equipment involved during thedark period beginning at time T3.

Refer now to FIGS. 3 and 4 which illustrate a preferred prismatic drycell battery useable in the battery charge system of FIG. 1. As thereillustrated the casing 4 comprises an open top housing body 4a ofgenerally rectangular configuration which may be made of insulatingmaterial or metal. The open top of the housing 4a is closed by a topwall 4b which is most advanta geously made of insulating material. Thetop wall of the housing carries the positive terminal the negativeterminal 12 and the control terminal 18, which may be screw terminals towhich the external circuits are suitable connected. The terminals 10, 12and 18 have conductive portions 10 and 12' and 18' extending through andbelow the top wall 4b. The battery plates 6 and 8 have a generallyrectangular configuration and are stacked in spaced parallel relation.The positive plates 6 have a series of connecting tabs 6a arranged inalignment and electrically connected in any suitable way to the positiveterminal extension 10' as by suitable connector means 27 engaging all ofthe tabs 6a. The negative plates 8 have similar aligned tabs 8a whichare connected by connector means 28 to the negative terminal extension12'.

The positive and negative plates 6 and 8 alternate in position and thelayers 14 of electrolyte impregnated separator material are sandwichedbetween the adjacent pairs of positive and negative plates. Theseparator layers may comprise a fibrous material, such as nylon, mattedinto a highly liquid absorbent body and may constitute a single lengthof separator material passing in zig-zag fashion between the variouspairs of plates and around the sides thereof. The separator layersproject a short distance beyond the normally top and bottom edges of theplates.

As illustrated, the oxygen consuming charge condition sensing electrode16 is a porous plate located in the middle portions of the stack ofpositive and negative battery plates where it has access to maximumconcentrations of electrolyte and water. The plate 16 may be positionedin place of one of the negative battery plates in the prismatic batteryso it is enveloped by an electrolyte wetted separator layer 14' andopposite one or more positive plates. It is connected by a conductivelink 35 to the conductive portion 18' of the control terminal 18extending through the upper wall 4b of the battery casing.

The platinum electrodes 19 are porous plates placed on the outside ofthe stack of battery plates where they have the best access to anyoxygen and hydrogen gases within the casing. Each of the porous plates19 is sandwiched between an outermost layer 33 of nylon netting orsimilar perforated insulating material, and an inner layer 14" ofelectrolyte absorbent material constituting a wick therefor. Theplatinum plates 19 have tabs 34 connected to the connector means joiningthe negative battery plates to the negative terminal 12.

The platinum group metal electodes may be platinum fuel cell electrodes,like the electrodes sold by the American Cyanamid Company with thedesignation AB6X. The electrodes found most suitable were 5 millimetersthick with about 8-10 milligrams of platinum black per squarecentimeters of the electrode mixed with graphite and Teflon particlespressed into a gold plated screen. The electrode porosity is preferablyin the range of about from 50-85 percent.

FIG. 6 shows the unexpectedly large hydrogen absorbing qualities of oneof the platinum electrodes 19 under charge conditions when connected tothe negative battery plates through a low impedance path so that thevoltage of the platinum electrode is substantially the same voltage asthat of the negative battery plates. It was found, for example, that attemperatures down to 30 degrees Centigrade, batteries like thatdescribed could be charged with a current as much as 10 or more timesthe ampere hour rating of the battery. Also, when the battery isoverdischarged, the current flowing through the battery can be as muchas one-half or more times the ampere hour rating of the battery.

The rate at which hydrogen is consumed at the platinum electrodes 19 isdependent upon the partial pressure of hydrogen as previously indicated.When the hydrogen gas pressure was artifially made 50-65 pounds persquare inch gauge, the consumption rate of each electrode 19 was about0.1 milliliters of hydrogen gas per milligram of platinum. Such hydrogengas absorption rate is entirely unexpected when the electrode isconnected to the negativebattery plates.

It should be understood that numerous modifications may be made in thepreferred forms of the invention described above without deviating fromthe broader aspects of the invention.

1 claim:

1. A dry cell battery system comprising: charge circuit means for thebattery, load circuit means for the battery, a sealed battery capable ofbeing repeatedly charged and discharged and including a sealed casinghaving exposed negative and positive terminals, and an exposed controlterminal; positive and negative battery plates within said casing andelectrically connected respectively to said positive and negativeterminals, said negative battery plates having a potential in the rangeof about minus 0.7 volts to about minus 0.9 volts, and an electrolytewithin said casing; means for connecting said charge and load circuit tosaid exposed positive and negative terminals; said battery forming withsaid positive and negative battery plates a rechargeable electrochemicalsystem which in conjunction with said circuit means generates oxygen atleas during the overcharge of the battery and hydrogen at rates farabove that absorbable by said battery plates; said high rate charging ofthe battery providing a progressively increasing amount of unconsumedoxygen which progressively builds up the pressure within the sealedbattery casing (a) as the fully charged condition of the battery isapproached, (b) during overcharge thereof and (c) for a while followingthe subsequent interruption of the charging thereof; porous oxygenconsuming electrode means within the casing electrically connected tosaid control terminal; current responsive means connected between saidnegative battery terminal and said control terminal; the porous oxygenconsuming electrode means effecting consumption of only a part of theoxygen generated during the charging of the battery, said oxygenconsumption resulting in the flow of current through said currentresponsive means connected between said negative and control terminalsof the battery, the value of which current progressively increases withthe amount of unconsumed oxygen and the consequent build-up of pressurein the casing; said current responsive means being responsive to theflow iii of current between said control and negative terminals at agiven trip level representing a desired fully charged condition of thebattery by operating said charge circuit means to terminate the highrate charging of the battery; and additional gas consuming electrodemeans in said casing comprising porous platinum group metal electrodemeans within said battery casing connected only to said negative batteryplates, said platinum group metal electrode means consumingsubstantially all the unconsumed oxygen remainingin the battery aftertermination of the battery charge at a relatively fast rate to bringsaid current flow in said current responsive means relatively rapidlybelow said trip level to enable the battery to be recharged again andalso consuming substantially all of said generated hydrogen.

2. The dry cell battery of claim 1, wherein said porous oxygen consumingcharge condition sensing electrode means is mounted within the stack ofpositive and negative battery plates and said platinum group metalelectode means is mounted on the outside of said stack of positive andnegative battery plates.

2. The dry cell battery of claim 1, wherein said porous oxygen consumingcharge condition sensing electrode means is mounted within the stack ofpositive and negative battery plates and said platinum group metalelecTode means is mounted on the outside of said stack of positive andnegative battery plates.